1. Field of the Invention
The invention relates to a cryogenic cooler for cooling an infrared detector employing a thermoelectric cooler and a Joule-Thomson cryostat.
2. Background and Summary of the Invention
Generally, infrared detectors must be cooled to extremely low temperatures to increase their sensitivity. Cryogenic coolers based on the Joule-Thomson effect have been used to cool detectors, however, when a limited cryogenic gas supply is available, for example in a gas reservoir in a missile or in a portable apparatus, the operating time of the cooler is limited by the available gas supply. Attempts to provide extended cooler operating times using thermoelectric (TE) coolers have been unsuccessful because TE coolers are unable to achieve the low temperatures necessary for operation of the detectors.
Other types of coolers have also been used to extend the operating time of infrared detectors including closed cycle coolers and internal gas compressors. These have resulted in coolers which are too expensive for most applications and are not able to cool to the low temperatures necessary for most detectors.
It has been suggested to combine a TE cooler and a Joule-Thomson (JT) cryostat by mounting a TE cooler at the base of a JT cryostat to regulate the temperature of incoming gas. In such an arrangement, when the TE cooler is mounted at the base of the JT cryostat, both the TE cooler and the JT cryostat are located within a dewar vessel. The dewar vessel including the TE cooler is mounted on a gimbal to permit the detector array to be aimed at a heat source. Mounting the TE cooler in the dewar vessel, however, limits the size and capacity of the TE cooler. TE coolers generate a significant amount of heat during operation and require means for disposing the heat, for example, a heat sink for efficient operation. The lack of a good heat sink when the TE cooler is mounted on a gimbal assembly limits the cooling capacity of the TE cooler and prevents any significant increase of the operating period for the system.
Another cryogenic cooling system employs two-stage JT coolers in which one cooling fluid is used in a first JT cooler for pre-cooling a second cooling fluid. The second cooling fluid is in turn used in a second JT cooler to cool a detector. This system requires two cooling gas supplies, and is accordingly more complex and expensive. This type of cooling system also has the disadvantage known in single stage cryogenic coolers of having limited gas supplies, and correspondingly limited operating periods.
A cooling system according to the present invention increases the operating period of a conventional Joule-Thomson cryostat cooler having a limited supply of cryogenic gas by adding a thermoelectric cooler to pre-cool the cryogenic gas. The present invention provides a TE cooler having a significant cooling capacity by locating the TE cooler on a heat sink remote from the JT cryostat. The TE cooler increases the cooling capacity of the cryogenic gas so that the cryogenic gas is conserved.
According to the present invention a cryogenic cooling apparatus is provided having a source of gas under pressure, a thermoelectric cooler for pre-cooling the pressurized gas, a dewar vessel, and a Joule-Thompson cooler within the dewar vessel. The TE cooler for pre-cooling the gas is in fluid communication with the source of pressurized gas. The JT cryostat is located within the dewar vessel and receives pressurized gas which has been precooled by the TE cooler at a location remote from the dewar vessel. The JT cryostat then employs the Joule-Thomson effect to cool a detector within the dewar vessel by expanding the pre-cooled pressurized gas. The JT cryostat operates more efficiently with the lowered inlet gas temperature and can extend the operating period of the JT cryostat when operating with a limited gas supply. The gas exiting the JT cryostat which is still cool is vented to a hot side of the TE cooler to aid in reducing the temperature of the TE cooler.
In accordance with another aspect of the invention, a missile seeker assembly includes a missile shell, a source of pressurized gas within the missile shell, a TE cooler and a dewar vessel within the missile shell. The TE cooler is in fluid communication with the source of pressurized gas to pre-cool the pressurized gas, The dewar vessel includes an infrared detector array and a JT cryostat for cooling the array. The JT cryostat receives the pre-cooled pressurized gas from the TE cooler and expands the pre-cooled gas to cool the detector,